The present invention relates to an electric motor, and particularly relates to an electric motor having magnetic flux detection elements for detecting changes in magnetic pole of a rotating magnet. The pole changes are detected to control an electric current flowing in toroidal coils wound on a stator core in accordance with detection signals of the magnetic flux detection elements.
FIG. 1 shows a conventional electric motor. The drawing shows the case in which the conventional electric motor is used for driving rotation of a polygon mirror of an optical deflector.
In FIG. 1, the electric motor includes a shaft 1, dynamic-pressure generating grooves 1--1, a housing 2, a rotary sleeve 3, a balance adjusting member attaching groove 4, a yoke 5, a magnet 6, a stator core 7, studs 8, a substrate 9, magnetic flux detection elements 10, a polygon mirror 11, a flange 12, screws 13, and a gap 15.
A rotor portion of the electric motor is mounted on the shaft 1 with the gap 15 therebetween. That is, the rotor portion is constituted by the rotary sleeve 3, the yoke 5 and the magnet 6 which are fixed to the rotary sleeve through pressing insertion, bonding, or the like, and further constituted by the polygon mirror 11 attached to the rotary sleeve 3 with the screws 13 through the flange 12.
On the other hand, a stator portion of the electric motor includes the housing 2, the shaft 1 fixed at its one end on the housing 2 through pressing insertion or the like, the stator core 7 similarly fixed to the housing 2 (being not shown, toroidal coils are wound on the stator core 7), the substrate 9 supported by the studs 8 attached to the stator core 7, the magnetic flux detection elements 10 planted on the substrate 9, and so on.
The magnet 6 is a permanent one, disposed so that a magnetic attracting force acts between the magnet 6 and the stator core 7 facing the magnet 6. The attracting force acts to prevent the opposite positions of the magnet 6 and the stator core 7 from shifting from each other in the axial direction of the electric motor (in the thrust direction).
That is, in FIG. 1, when the magnet 6 moves up, a component for drawing the magnet 6 down appears in the attracting force to thereby draw the magnet 6 down, while when the magnet 6 moves down, a component for drawing the magnet 6 up appears in the attracting force to thereby draw the magnet 6 up. Thus, the magnet 6 and the stator core 7 are kept in opposition to each other at a predetermined axial position by the magnetic attracting force. That is, a magnetic thrust bearing includes the magnet 6 and the stator core 7.
For example, Hall elements may be used as the magnetic flux detection elements 10. Each Hall element detects leakage flux of the magnet 6 to thereby detect whether an N pole or an S pole passes by the Hall element when the magnet 6 rotates.
Detection signals of the respective Hall elements are sent to a not-shown control portion through wirings printed on the substrate 9. On the basis of the detection signals, the control portion determines the direction of a current to be made to flow into the toroidal coils which are wound on the stator core 7 at various positions thereof. As a result, force is generated in the direction to maintain continuous rotation of the magnet 6, by the mutual operation of the magnet 6 and the stator core 7.
A radial bearing is a bearing for preventing the center of the rotation from shifting from a predetermined position even if force is exerted onto the shaft in the direction perpendicular to the shaft. In the electric motor shown in FIG. 1, a dynamic-pressure air bearing is used as the radial bearing. The dynamic pressure is generated by the dynamic pressure generating grooves 1--1.
When the rotary sleeve 3 rotates, a high-pressure air layer is generated by the dynamic-pressure generating grooves H around the shaft 1 (the portion of the gap 15). The rotary sleeve 3 is supported by the pressure in the state where the rotary sleeve 3 is floating from the shaft 1.
Although the dynamic-pressure generating grooves are formed in the shaft 1 at its outer circumference in the example, the dynamic-pressure generating grooves may be formed in the rotary sleeve 3 in its inner wall.
The air layer operates so as to maintain the center of rotation of the rotor portion. For example, if the rotary sleeve 3 shifts right in FIG. 1, the right gap is made larger so that the pressure in the gap at this position becomes lower than that before the rotary sleeve 3 shifts. At this time, the left gap becomes smaller so that the pressure in the gap at this position becomes higher than that before the rotary sleeve 3 shifts. When the above-mentioned relation of the degree of pressure is established, the rotary sleeve 3 is pushed left so that the rotary sleeve 3 is finally returned to the original position.
The polygon mirror 11 has a polygonal shape when viewed from above in the axial direction, and the polygon mirror 11 has a number of mirror surfaces on its circumferential side surfaces, the mirror surfaces being irradiated with a light beam such as a laser or the like. If the polygon mirror 11 is rotated while the first mirror surface is being irradiated with a light beam, the direction of the light beam reflected from the first mirror surface is gradually changed. That is, the reflected light beam is deflected.
When the polygon mirror 11 is further rotated so that the first mirror surface is not irradiated with the light beam, a second mirror surface rotates to a position where it is irradiated with the light beam. A this time, deflection is performed by the second mirror surface similarly to that by the first mirror surface. Thus, the reflected light beam performs scanning within a range of a fixed angle. The scanning speed depends on the rotation speed of the polygon mirror 11.
However, one problem of the conventional electric motor described above is that much noise flux enters into the magnetic flux detection elements for detecting changes in magnetic pole of the rotating magnet so that S/N (the signal-to-noise ratio) is poor.
FIG. 2 is an enlarged view showing a peripheral portion of each of the magnetic flux detection elements of the conventional electric motor. The parts the same as or equivalent to those in FIG. 1 are referenced correspondingly. The reference numeral 16 designates a toroidal coil, 17, magnetic flux leaking from the toroidal coil, 18, magnetic flux passing through the stator core, and 19, magnetic flux of the magnet.
Conventionally, the magnetic flux detection elements 10 are provided in the lower middle portion between the magnet 6 and the stator core 7 so as to detect the magnetic flux 19 going to the stator core 7 from the magnet 6.
When a current is made to flow in the toroidal coils 16, magnetic flux is generated. Assuming that a current flows in the toroidal coils 16 in the direction shown by arrows, then a part of the generated magnetic flux passes, as the stator-core-passing-through magnetic flux 18, through the stator core 7 of a magnetic substance, and the remainder of the generated magnetic flux leaks out into the space as the toroidal-coil-leakage magnetic flux 17.
Also the toroidal coil leakage magnetic flux 17 is detected by the magnetic flux detection elements 10. However, the toroidal coil leakage magnetic flux 17 is not an object to be detected by the magnetic flux detection elements 10 but a mere noise component.
Thus, the ratio S/N of the magnetic flux detection elements 10 becomes poor under the influence of the toroidal coil leakage magnetic flux 17. Therefore, the changes in magnetic pole of the magnet 6 can not be detected with high accuracy.